1. Field of the Invention
The present invention relates to an image forming apparatus for use in obtaining a high-quality record image, and more particularly to image correction in image formation using an electrophotography architecture.
2. Description of the Related Art
Today, a digital electrophotographic device using an electrophotography process and a laser scanning technology for use as an output terminal of a digital equipment, such as a personal computer and a workstation or the like, is becoming more important in the manufacture of printers due to superiority including accelerated recordability and enhanced print quality. Especially, as demands for color visualization of computer-produced documents increase in recent years, a complete or full color-generatable image forming apparatus becomes available in the market.
A conventional image forming apparatus using an electrophotography technique will be explained hereinafter. FIG. 8 is a diagram showing an overall configuration of a conventional color image forming apparatus using electrophotography. Reference numeral 8 designates a photosensitive drum. The photosensitive drum 8 consists essentially of an aluminum drum, charge-generating layer (CGL), and charge transfer layer (CTL). The photosensitive drum 8 is driven by a drive motor (not shown) to rotate in a specified direction designated by arrow "A".
There are provided on the circumferential surface of the photosensitive drum 8 an electrification device or electrifier 9, exposing optical system 10, development rotary unit 12, and intermediate transfer body 13, which are disposed in the order of sequence along the rotation direction indicated by arrow "A". The development rotary unit 12 is provided with developing devices 11K, 11Y, 11M, 11C of respective colors such as black (K), yellow (Y), magenta (M) and cyan (C), which are operable to rotate once per print cycle of each color to come into circumferential contact with the photosensitive drum 8 for development.
The electrifier 9 is structured including an electrically chargeable brush (not shown) made of rayon. The electrifier 9 is brought into direct contact with the surface of the photosensitive drum 8 upon application of a negative voltage (approximately -1 KV) to the chargeable brush to thereby cause the surface to be uniformly electrified or charged at a negative potential of -600 V, or more or less.
The exposing optical system 10 is configured from a laser drive device, polygon mirror, lens system, polygon mirror rotating motor (scanner motor) and others (not shown). The exposing optical system 10 is operable to optically modulate and optically scan image data as indicated by arrow "B" to thereby form it as an electrostatic latent image on the charged surface of the photosensitive drum 8.
Each of the developing devices 11K, 11Y, 11M, 11C houses a development roller 14 using black, yellow, magenta or cyan toner particles and a conductive rubber, and a thin-layer roller 15. The development rotary unit 12 rotates in the arrow "C" direction once per print cycle of each color, so that each color developer comes into contact with the photosensitive drum 8.
In the initial print cycle, when the development rotary unit 12 is driven to rotate up to a specified position at which the black developer 11K is contacted with the photosensitive drum 8. Simultaneously, the development roller 14 is driven to rotate in the forward direction with respect to the arrow "A" direction of the photosensitive drum 8 at that position. Thereby, black toner particles thin-layered from the inside of a corresponding developer by the thin-layer roller 15K are supplied to the surface of the development roller 14K. The black toner is charged negatively due to friction at a time point of such thin-layer process.
A surface potential (bright potential) of the part of the surface of the photosensitive drum 8 at which the electrostatic latent image is created increases ranging between -50 to -100 V or therearound. Thereby, when a negative voltage of about -300 V is applied to the development roller 14K, an electric field is generated in the direction of from the photosensitive drum 8 toward the development roller 14K. As a result, the Coulomb force acts on the black toner, which is negatively charged on the development roller 14K, in the reverse direction of such electric field, i.e. in the direction of the photosensitive drum 8. Thereby, the black toner is attached to the latent image part formed on the photosensitive drum 8.
On the other hand, the remaining part on the surface of the photosensitive drum 8 at which any electrostatic latent image is not formed is -600 V in surface potential (dark potential). As a result, no electric field is created in the direction from the development roller to the photosensitive drum 8 even upon application of a development bias voltage thereto, so that no black toner is attached to the photosensitive drum 8. The aforementioned development process is generally called the "negative-positive process" or "inversion phenomena" due to the fact that toner is attached to only the part to which light is irradiated (namely, white).
Next, a positive intermediate transfer bias voltage of about 500 V is applied to the intermediate transfer body 13 to produce an electric field in the direction from the intermediate transfer body 13 to the photosensitive drum 8. As a result, the Coulomb force acts on the black toner, which is negatively charged on the photosensitive drum 8, in the reverse direction of such electric field, i.e. in the direction of the intermediate transfer body 13. Thereby, the black toner is transferred to the intermediate transfer body 13. The intermediate transfer body 13 is composed of a metal drum base tube which is made of aluminum or the like, and a belt-like sheet which is made of conductive resin or the like and is wound around the tube. The intermediate transfer body 13 is driven by a drive motor (not shown) to rotate in the arrow "D" direction.
When the black toner transfer is completed, the development rotary unit 12 rotates in the next print cycle, so that the cyan developer 11C comes into contact with the photosensitive drum 8. As in the black developer 11K, the cyan developer 11C operates to develop a cyan toner image on the photosensitive drum 8, and then the cyan toner is transferred from the photosensitive drum 8 to the intermediate transfer body 13. In this way, mono-color printing is repeatedly carried out on the intermediate transfer body 13 with respect to four colors in total to thereby form a lamination of four-color image components superimposed on one another. As a result, a full color image is formed.
When such color lamination on the intermediate transfer body 13 is completed, a sheet of paper 17 in a paper cassette 16 is transported into the main body of the apparatus by a paper-feed roller 18. When the paper sheet 17 is transported at a location corresponding to the intermediate transfer body 13 and transfer roller 19, a positive transfer bias voltage of about 1 KV is applied to the transfer roller 19. Thereby, an electric field is created in the direction of from the transfer roller 19 toward the intermediate transfer body 13. As a result, the Coulomb force acts on the color-stacked toner, which is negatively charged on the intermediate transfer body 13, in the reverse direction of the electric field, i.e. in the direction of the transfer roller 19. Thereby, color-superimposed toner image is transferred to the paper sheet 17. Further, a cleaning unit 23 comes into contact with the intermediate transfer body 13 at the same time when the image transfer is performed, so that any residual toner particles on the intermediate transfer body 13 are collected.
Lastly, the toner image that has been transferred to the paper sheet 17 is fixed on the paper sheet 17 by a fixing device 22, which includes a heat roller 20 being temperature-controlled by a halogen lamp (not shown) at a temperature of about 160.degree. C. and a pressing roller 21, and is then outputted as a full color image.
On the other hand, as shown in FIG. 9, the image data and the outputted image density exhibit a typical input-output characteristic (referred to as "gamma characteristic" hereinafter) of the electrophotography process. Since this characteristic is not linear in the image density (tone property) with respect to a change in the density of the image data, the required image density is hardly obtainable at low-density (high light) portions, so that the image density is saturated at high-density portions. Therefore, the gamma correction, in which the image density on this intermediate transfer body 13 is detected by a density detector unit 24 and then the gamma characteristic is corrected into linearity, is employed in order to achieve the good continuous shading gradations with respect to the image data.
Here, the gamma correction will be explained. FIG. 10 is a conventional gamma correction control block diagram. In FIG. 10, reference numeral 4 designates an image processor unit for processing print data from a host computer (not shown) to output it as image data, reference numeral 5 designates a test pattern generator unit for generating test pattern image data shown in FIG. 11 in order to effect the gamma correction, and reference numeral 6 designates a corrector unit for converting the image data, which is outputted from the image processor unit 4 and test pattern generator unit 5, into it having the steady image density. More practically, the density detection of the test pattern image data outputted from the test pattern generator unit 5 is performed. The gamma correction, which is based on the correction data such as a look-up table or the like, is performed. Thereafter, the gamma correction of the image data outputted from the image processor unit 4 is performed on the basis of the correction data such as a lookup table or the like again.
Reference numeral 7 designates a modulator unit for optically modulating a semiconductor laser (not shown) of the exposing optical system 10 by the image data after conversion outputted from the corrector unit 6; reference numeral 24 designates a density detector unit for detecting by reflection light the density of the image data on the intermediate transfer body 13, which is outputted from the test pattern generator unit 5; and reference numeral 25 designates a control unit for inputting the image density data outputted from the density detector unit 24, for arithmetically processing the inputted image density data in a way such that the image data and the image density exhibit linearity, and for outputting the processed data to the corrector unit 6.
Next, the gamma correction will be explained. FIG. 12 is a flow chart of a conventional gamma correction sequence, FIG. 13A is a gamma characteristic diagram representing a relation of image data versus image density, and FIG. 13B is a gamma characteristic diagram representative of a relation of the image data versus image density after the gamma correction.
In the flow chart of FIG. 12, the initialization of a laser beam printer (LBP) is started after power-up (step 10). At this step, it is set whether or not the gamma correction is performed in an input unit (not shown) such as a front panel, and at the control unit 25 it is determined whether or not the gamma correction is performed (step 20). If the gamma correction is performed, then the control unit 25 controls the drive timing of each LBP to output a print signal to the test pattern generator unit 5.
As shown in FIG. 11, the test pattern generator unit 5 sequentially outputs the test pattern image data for black (BK) (step 30). The image data may be 8-bit data composed of 15 kinds of image data items: 11 (HEX), 22 (HEX), 33 (HEX), 44 (HEX), 55 (HEX), 66 (HEX), 77 (HEX), 88 (HEX), 99 (HEX), AA (HEX), BB (HEX), CC (HEX), DD (HEX), EE (HEX), and FF (HEX).
The outputted test pattern image data is converted by a conversion table (not shown) for black in the corrector unit 6, and then the modulator unit 7 optically modulates the semiconductor laser in the exposing optical system 10. Here, the conversion table is in one-to-one correspondence in the initial values thereof. The modulated test pattern image data becomes a black toner image on the intermediate transfer body 13, and then the density of such black image (toner) on the intermediate transfer body 13 is detected by the density detector unit 24 (step 40).
Here, the image density detected by the density detector unit 24 generally exhibits the characteristic as shown in FIG. 13A. In order that the control unit 25 corrects, as shown in FIG. 13B, the gamma characteristic into linearity based on this characteristic, the inverse function before correction is set in the black conversion table (not shown) within the corrector unit 6 (step 50). Prior to entering the next step, the surface of the intermediate transfer body 13 is cleaned by the cleaning control unit 23 to remove any residual toner powders away therefrom.
Similarly, the test pattern image data for cyan (C) is sequentially outputted (step 60), and then the cyan image (toner) density on the intermediate transfer body 13 is detected by the density detector unit 24 (step 70). In order to correct the gamma characteristic into linearity as shown in FIG. 13B, the inverse function before correction is set in a cyan's conversion table (not shown) within the corrector unit 6 (step 80). Prior to proceeding to the next step, the surface of the intermediate transfer body 13 is cleaned by the cleaning control unit 23 to remove residual toner particles away therefrom.
Similarly, the test pattern image data for magenta (M) is sequentially outputted (step 90), and then the magenta image (toner) density on the intermediate transfer body 13 is detected by the density detector unit 24 (step 100). In order to correct the gamma characteristic into linearity as shown in FIG. 13B, the inverse function before correction is set in a magenta's conversion table (not shown) within the corrector unit 6 (step 110). Prior to entering the next step, the surface of the intermediate transfer body 13 is cleaned by the cleaning control unit 23 to remove residual toner particles therefrom.
Similarly, the test pattern image data for yellow (Y) is sequentially outputted (step 120), and then the yellow image (toner) density on the intermediate transfer body 13 is detected by the density detector unit 24 (step 130). In order to correct the gamma characteristic into linearity as shown in FIG. 13B, the inverse function before correction is set in a yellow's conversion table (not shown) within the corrector unit 6 (step 140). Before going to the next step, the surface of the intermediate transfer body 13 is cleaned by the cleaning control unit 23 to remove residual toner particles away therefrom. In this way, the gamma correction processes for four colors are completed, and then the LBP is in the standby (on-line) state.
However, in the conventional gamma correction mentioned above, there is a problem that the image quality is reduced because it is impossible to realize the good continuous color shading gradations for the following reason. As the gamma correction is performed on the intermediate transfer body 13, when the real image density data is transferred and outputted to a sheet of paper, it is impossible to correct to the linearity as shown in FIG. 14 due to a change in transfer efficiency originated from the image density and any possible environmental variations or the like.